Manufacture of ceramic articles



rea it... 4. 19 3s acme 2,10%.609 MANUFACTURE or case narrows Peter de Leeuw, Niagara Falls, N. E, assignor, by mesne assignments, to The Carborundum Comparry, Niagara Falls, N. Y... a corporation oi Delaware pplication February 9, 1935, Serial No. ates This invention relates to the production 6f small voids. The size of the voids in an article porous ceramic bodies by the puddled method and made by the gas method is ailected by the proparticularly to theproduction of ceramic bonded portion of gas evolving substance in the mix and abrasive articles by the said method. when a relatively large proportion of such sub- In making porous and/or cellular ceramicarstance is added to the mixto obtain an article 5 ticles and particularly abrasive articles by the having a high cellularity the voids produced in puddled method two alternative methods are at the article are relatively large in size. Thetendpresent employed for controllably increasing the ency of the mix to fall when using the gas method porosity and/or cellularity of the articles when is intensified when a large proportion of .gas

No Drawing.

' m finished. I evolving substance is used inan attempt to obtain 10 One of these methods comprises the addition an article of great cellularity. 1 ii of gas evolving substances such as peroxides, car- Furthermore when the gas method is used a bonates, etc. to the mix and allowing them to demore or less impervious skin or scum is formed compose while the mass is still in a plastic con-'- over the top of the article while it is being dried.

l5 dition. The gas evolved by the decomposition of This slows up the rate of drying and causes un- 1 the added substances causes the formation of even drying with resultant cracking.

voids and, depending upon the skill with which When combustible particles such as granulated the mix is handled, these voids will remain until coke are used in a puddled mix, the coke tends the mix'sets sufliciently. The green article is to float to the top of the wheel after the mix is then dried and fired to produce a vitrified article poured into the mold. The cokehas a further go. having a cellular structure substantially the same tendency to ball together in the mix causing the as that of the green article. la production of undesirably large voids when the The other method of producing and controlling wheel is' burned. I the structure of ceramic bodies made by the pudwhen it is attempted to produce an article of v died method comprises addingto the mix combusrelatively high porosity by the addition of coke g5 tible particles such as'coke. etc. which will be to the mix there is a tendencyior part of the burned out when thearticle is being fired and coke to remain unburned thus defeating the purwill leave voids corresponding in shape and size pose of its addition and resulting in an article of to those of the combustible particles used. nonuniform porosity. g The voids which are formed by the evolution It is an object of the present invention to pro- 30 of gas are different in shape than those obtained vide a method of making porous ceramic articles I by the burning out of combustible particles. As whereby the disadvantages of the prior methods aresult the articles produced by the two methods set forth above are obviated, and to produce a have difierent properties. ceramic structure having new and-unusual chara- The evolution of gas produces a structure with acteristics. v a preponderance of discrete voids. The voids The present invention comprises the addition are only occasionally interconnected and each to a mix for making a ceramic articleby the one is in the majority of'caseseompletely enclosed puddled method, gas evolving substances and ma shell of bond and grain. An article made comminuted combustible particles,controlling the o by-such a method might be defined as comprisevolution of gas, in the mix before drying to pro- 40 ing a plurality of adhering shells of grain and duce a greenarticlehaving a cellular structure bond, the interior of the said shells interconnectand then burning out the combustible particles ing with adjacent shells only occasionally. on during the firing of the article to produce a vitri-' the other hand the method utilizing-the comfied article having relatively great porosity.

bustible particles such as coke produces porous As gas evolving material and combustible ma- 45 articles having a plurality of irregularly shaped 'terial any of thesubstances previously used alone voids which are' substantially interconnected. for this purpose may be used in the combined Two articles made by the said methods might be method of the present invention. identical in all respects including density but- In addition the stabilizing agent and catalysts the article made by the burning out of combustipreviously used in connection with gas evolving so ble particles would be more permeable than-the substances tocontrol the rate or degree of evoluarticle'made by. the evolution of gas. tion of gas may be usedin the new method.

When using the gas method it is difflcult if A great variety of materials and combinations not impossibleto obtain anarticle' having a relof materials may thus be used in the presenta a lv y high cellularity made up of relatively invention but the'inventlon will be further described in detail in connection with two mateg is about 1.60 and the porosity advantageous; that is, coke as the comminuted combustible material and hydrogen peroxide as the gas evolving material.

While the'invention is applicable to the production of porous ceramic articles generally, it

will be further described with particular reference to the production of a porous abrasive article. The invention may be carried out according to the following example which illustrates one modification of the invention.

Example 4500 grams of abrasive grain, such as fused alumina, 1500 grams of clay bond and 300 grams of screened coke preferably petroleum coke are mixed together by any suitable means to produce an intimate mixture. The abrasive grain may be of any suitable size and it may be desirable to control the size of the screened coke with reference to the size of the abrasive grain. For example, the abrasive grain of the above mixture may be selected as grit and the coke may be a size which will pass through an 82 mesh screen and be retained by a 109 mesh screen.

16 grams of crystalline sodium pyrophosphate and 1 gram of sodium stannate are dissolved in 1200 cc. of water and added to the dry mix described above. The water solution and the dry ingredients are intimately mixed and then 300 cc.

of a 2 glue solution are added and intimately mixed in. cc. of 4 hydrogen peroxide solution are then added to the mix and the whole mixed just long enough to incorporate the hydrogen peroxide in themix uniformly.

The mix or slip is then placed in a suitable mold and allowed to dry. The hydrogen peroxide will decompose before the mix sets and will form a cellular green article.

The dried article is then shaved and cut to a suitable size and placed in a kiln to be burned. During the burning the coke is oxidized to a gas which escapes, resulting in the formation of voids in addition to those formed by the solution of the hydrogen peroxide. The density of a product made in accordance with the above example "The structure of the article made above, according to the present invention is distinctly difierent from that of articles made in accordance with previous methods. When a substantial proportion of gas evolving substance such as the hydrogen peroxide is used, the dried green article, before firing, will be a matrix defining a plurality of discrete voids substantially rotund in shape, that is, spherical, oblate, 0r prolate. When the article is fired the coke will be burned out leaving a plurality of additional voids. These voids will be somewhat irregular in shape following roughly the original shape of the grain of coke which occupied the space. Due to the fact that the grains of coke generally are in contact with other grains of coke throughout the mix, or are adiacent a bubble of gas formed by the evolution of oxygen from the hydrogen peroxide the article will have after the firing thereof a permeable structure in distinction to the somewhat impermeable structure before burning, that is, the voids will substantially all interconnect with at least one adjacent void, with the result that the article will be permeable. The degree of porosity depends upon the proportion of. void producing substances used in the mix and the degree of permeability depends roughly upon the proporthe present invention the porosity tion of coke used to produce the porosity. The finished article made according to the invention may be said to consist of a matrix of ceramic material which may be a mixture of bond and grain, the matrix defining a plurality of substantially rotund voids interconnected by a plurality of irregular shaped voids. The expression "rotund is used generally and here specifically to describe shapes partaking of the nature of a fluid bubble which may or may not have been acted upon or constricted by surrounding forces. Such a pore in a ceramic article may be spherical in shape or it may be oblate, prolate or otherwise deformed from the spherical shape but will have relatively smooth walls and the characteristic appearance of a void caused by the expansion of gas.

As stated above, other gas evolving substances than hydrogen peroxide may be used. For example, sodium peroxide, sodium perborate, ammonium persulphate, urea peroxide and other peroxides and per-salts. Instead of coke other granular combustible material may be used such as wood flour, sawdust. seeds and grits obtained from seeds.

The hydrogen peroxide concentration may be varied within wide limits. In the manufacture of abrasive articles very good results are still obtainable even if the proportion oi the hydrogen peroxide is doubled that used in the example given above. There is no lower limit for the hydrogen peroxide concentration. Small quantities not having any apparent efl'ect on the pore structure may be used for the purpose of deflocculating the mass and to assist in keeping the grain and coke particles in suspension and preventing them from settling out, or balling. The evolution of gas from the decomposition of hydrogen peroxide increases the viscosity of the mix and this further reduces the tendency of the ingredients of the mass to settle out.

The addition of sodium pyrophosphate and sodium stannate to the mix serves to regulate the decomposition of the hydrogen peroxide or other per compounds used to evolve gas and they have the additional action of deflocculating the ingredients oi' the mix. These ingredients tend to separate the coke into individual particles and thus assist the formation of voids of uniform size. The amount of the coke may be varied up to about 50% or more of the weight of the grain and bond.

The invention has been specifically described with reference to the manufacture of porous abrasive bodies but as stated above the invention may also be applied to the production of other articles, as for example, filter plates, refractory materials 'such as insulating brick and similar articles. The present invention is of particular use in the manufacture of filter plates having a consistently uniform permeability. By means of and pore size can be accurately controlled. The coke particles serve to puncture the film of bond surrounding each void in the cellular structure formed by the gas evolving substances. The permeability of an article can therefore be independently controlled by regulating the absolute and relative amount of combustible particles and gas evolving substance used in the mix.

It is thus seen that the two void producing materials added to the mix cooperate to produce beneficial results in the formation of the article and in the article itself which it is not possible to obtain by means of the use of one of them alone. 76

By means of the present invention it is pos-- By means of the present invention it is possible to obtain a structure of novel physical characteristics and in addition a structure of lower density than may be obtained by the use of the gas method alone, because in the gas evolving method there is a tendency for the body to fall before it becomes rigid, especially so if much peroxide is added.

Furthermore by means of the present invention it is possible to obtain a puddled article having'a high porosity made up of small pores. This is an advantage over the gas method alone because in the latter method a relatively large amount of gas evolving substance is added to the mix in an attempt to obtain a, greater porosity or cellularity. The number of voids formed will be substantially the same but the size will be increased to produce the greater cellularity. Without the use of peroxide or other gas evolving substance, the coke has a tendency to float to the top of the puddled wheel thus producing a finished article having a non-uniform distribution of pores. When gas evolving materials are used in the mix together with the coke this difficulty of the separation of the coke does not occur, probably because of the more viscous nature of the gassy mix.

When coke is used in connection with the peroxide method the impervious skin characteristic of the peroxide method does not occur with the result that the surface of the wheel remains pervious enabling it to dry quicker and more evenly.

When the two types of void reducing materials are used in conjunction as in the present invention the tendency of part of the coke to remain unburned when that substance is used alone is obviated. This is apparently because the article is already somewhat cellular when it is put in the kiln for firing due to the previous formation of voids by the action of the gas evolving material. The cellular nature of thefgreen article undoubtedly assists in the complete burning out or the coke during the firing of the article.

sible to produce articles by the puddle method having a controlled pore size and distribution. Articles made by this methodare exceptionally uniform in the distribution of the. pores. Furthermore by this method it is possible to produce articles having the advantages of the spherical or rotund voids produced by the generation of the gas and the substantial permeability due to the burning out of the combustible particles. By means of the invention it is possible therefore to produce an article having an exceptionally uniform porosity with pores of the desired'size and with the desired degree of permeability.

I claim:

1. Ihe method of making a porous abrasive article, which comprises forming a slip comprising abrasive grain and bond, incorporating in the slip granular combustible material and gas evolving material, placing the slip in a mold to permit the gas evolving material to decompose, drying the article and burning it in a kiln to vitrify the bond and burn out the granular combustible material.

2. The method of making a porous abrasive article which comprises forming a slip comprising abrasive grain and bond, incorporating in the mix granulated coke and hydrogen peroxide, placing the mix in a mold to permit the expansion of the article by the evolution of oxygen from the decomposition of the hydrogen peroxide, drying the expanded article and burning it in a kiln to vitriiy the bond and burn out the coke particles.

- 3. The method of making a porous ceramic article which comprises forming. a puddling slip containing granular material, a binder therefor, water, granular combustible material and gasevolving material.

4. The method of making a porous ceramic article which comprises forming a puddling slip containlngxg'ranular combustible material and s material. v 

